5-aminolevulinic acid (ALA) is a precursor of haem biosynthesis and its synthesis is a rate-limiting step in this pathway. When ALA is supplied to certain cells exogenously, protoporphyrin IX is accumulated in the cells because conversion of protoporphyrin to haem by ferrochelatase becomes rate-limiting (Malik, Z. and M. Djaldetti, Cell Different. 8:223-233 (1979)). Because protoporphyrin is a photosensitizer, subsequent exposure to light leads to cell destruction, primarily by damage to the mitochondria (Linuma, S., et al., Br. J. Cancer 70:21-28 (1994)).
Photodynamic therapy mediated by ALA was proposed in 1990 as a new cancer treatment (Kennedy, J. C., et al., J. Photochem. Photobiol. B:Biol. 6:143-148 (1990)). Topical application of ALA followed by exposure to light has been used successfully for eradication of various skin cancers in clinical studies (Kennedy, J. C. and R. H. Pottier, J. Photochem. Photobiol. B:Biol. 14:275-292 (1992); Fijan, S., et al., Br. J. Dermatol. 133:282-288 (1995); Roberts, D. J. H. and F. Cairnduff, Br. J. Plastic Surg. 48:360-370 (1995)). In addition, the combination of ALA and light has been suggested for treating mycosis fungoides (Wolf, P., et al., J. Am. Acad. Dermatol. 31:678-680 (1994)), as well as the ablation of the endometrium as an alternative to hysterectomy or for sterilization (Yang, J. Z., et al. Am. J. Obst. Gynecol. 168:995-1001 (1993)). However, prior to the present invention, the combined use of ALA and light has not been demonstrated to be useful for inactivating intracellular viruses either in vitro or in vivo.